Automotive on-board power supply systems usually include a battery and a generator. A plurality of electrical loads may be coupled to the on-board power supply system. These loads can include a starter, electrical motors, and lights, as well as control and driver units for driving the individual loads. Most of the electrical loads connected to an on-board power supply system require a supply voltage with a given polarity and must not have the polarity of their supply voltage reversed. Each of these loads may include a reverse polarity protection circuit or element, like a diode. This, however, increases the costs of the overall system.
While in modern cars it is almost impossible to connect the car battery with a reverse polarity in the on-board power supply system, a reverse polarity error may occur when the on-board system is connected to an external power source, like a battery charger or the battery of another car. Connecting the battery to an external source can be required for charging the battery or for jump-start purposes. In modern cars, terminals for connecting an external power source are located remote from the battery and are connected to the on-board systems through electrically connecting wires. These terminals are also known as jump-start terminals or assist start terminals.
Although these terminals are labeled in a manner which should prevent a reverse polarization, errors may occur.